It is expected to move across parts of southeastern North Carolina and eastern SC on Friday and Saturday, then head north over the western Carolinas and central Appalachian Mountains early next week, the NHC said.
President Donald Trump touted the government's readiness and urged people to get out of the way of Florence. "It's a big one", he said.
Weather Underground meteorology director Jeff Masters said Florence eventually could strike as merely a Category 1 hurricane with winds less than 100 miles per hour (160 kph), but that's still enough to cause at least $1 billion in damage.
The storm was heading for the coast of the two states but heavy rain was also expected in Virginia to the north and Georgia to the south.
Forecasters said wind speeds have dropped from a high of 140 miles per hour (225 kph) to 110 miles per hour (175 kph), reducing it from a Category 4 storm to a Category 2, and additional fluctuations and weakening were likely as it swirled toward land.
More than 440,000 homes and businesses were without power in North and SC yesterday, utility officials said.
Besides inundating the coast with wind-driven storm surges of seawater as high as 13 feet (four meters) along the Carolina coast, Florence could dump 20 to 30 inches (51-76 cm) of rain, with up to 40 inches in parts of North Carolina, the NHC said.
WILMINGTON (North Carolina) • Hurricane Florence, weakened but still unsafe, crashed into the Carolinas yesterday as a giant, slow-moving storm that stranded residents with flood waters and swamped part of the town of New Bern at the beginning of what could be a days-long deluge.
About 1.7 million people have already been told to leave their homes in the latter three states, but some who thought they were safely out of the path of the storm are now desperately working to board up their homes and businesses before its arrival.
Storm surge has accounted for about half of the deaths in hurricanes since 1970, according to the National Hurricane Center.
Duke Energy, the second-largest energy company in the United States, said Florence could knock out electricity to three-quarters of its four million customers in the Carolinas. Utility companies said millions were expected to lose power, and that restoring it could take weeks.
Computer models of exactly what the storm might do varied, adding to the uncertainty.
"I ask all Georgians to join me in praying for the safety of our people and all those in the path of Hurricane Florence", Mr Deal said.
"We have crews sheltered in place at this time, waiting for the storm's passage", said Capt. Bion Stewart, the Coast Guard incident commander for Hurricane Florence.
Ricardo Olivares, of Wilmington, North Carolina, covers windows at the Mellow Mushroom in Wilmington, North Carolina on September 11, 2018. Some of those who stayed went to shelters while others stuck it out in their homes.Maysie Baumgardner, 7, and her family sheltered at the Hotel Ballast in downtown Wilmington as Florence filled the streets with floodwaters.
"We're just trying to plan for the future here, not having a house for an extended period of time", David Garrigus said. The storm will arrive on the East Coast sometime Thursday and linger for several days, forecasters say.
Early Thursday Florence was about 235 miles (378 kilometers) east southeast of Wilmington, North Carolina and about 280 miles (450 kilometers) east southeast of Myrtle Beach, South Carolina. Most other beachgoers were long gone.